Content

• Steps
• Basic information and definitions
• Method 1 of 2: Presentation of monohybrid crosses (one gene)
• Method 2 of 2: Introducing a dihybrid cross (two genes)
• Tips
• Warnings

The Pennett grid is a visual tool that helps geneticists identify possible combinations of genes during fertilization. A Punnett lattice is a simple table of 2x2 (or more) cells. With the help of this table and knowledge of the genotypes of both parents, scientists can predict what combinations of genes are possible in offspring, and even determine the likelihood of inheriting certain traits.

Steps

Basic information and definitions

To skip this section and go directly to a description of the Punnett Lattice, click here.

1. 1 Learn more about the concept of genes. Before you start learning and using the Pennett Lattice, you should be familiar with some basic principles and concepts. The first such principle is that all living things (from tiny microbes to giant blue whales) have genes... Genes are incredibly complex microscopic sets of instructions that are embedded in virtually every cell in a living organism. In fact, to one degree or another, genes are responsible for every aspect of an organism's life, including how it looks, how it behaves, and much, much more.
   • When working with a Pennett lattice, one should also remember the principle that living organisms inherit genes from their parents... You may have subconsciously understood this before. Think for yourself: is it not in vain that children, as a rule, look like their parents?
2. 2 Learn more about the concept of sexual reproduction. Most (but not all) of the living organisms you know produce offspring through sexual reproduction... This means that the male and female contribute their genes, and their offspring inherit about half of the genes from each parent.The Punnett lattice is used to graphically depict different combinations of genes of parents.
   • Sexual reproduction is not the only way to reproduce living organisms. Some organisms (for example, many types of bacteria) reproduce themselves through asexual reproductionwhen offspring is created by one parent. In asexual reproduction, all genes are inherited from one parent, and the offspring is almost an exact copy of it.
3. 3 Learn about the concept of alleles. As noted above, the genes of a living organism are a set of instructions that tell each cell what to do. In fact, just like the usual instructions, which are divided into separate chapters, clauses, and subclauses, the different parts of the genes indicate how different things should be done. If two organisms have different "subdivisions", they will look or behave differently - for example, genetic differences could cause one person to have dark hair and another to have blonde hair. These different types of one gene are called alleles.
   • Since the child receives two sets of genes - one from each parent - he will have two copies of each allele.
4. 4 Learn about the concept of dominant and recessive alleles. Alleles do not always have the same genetic "strength". Some alleles called dominant, will certainly manifest themselves in the child's appearance and behavior. Others, so called recessive alleles appear only if they do not mate with the dominant alleles that "suppress" them. The Punnett grid is often used to determine how likely a child is to receive a dominant or recessive allele.
   • Since the recessive alleles are "suppressed" by the dominant ones, they appear less frequently, in which case the child usually receives the recessive alleles from both parents. Sickle cell anemia is often cited as an example of an inherited feature, but it should be borne in mind that recessive alleles are not always "bad".

Method 1 of 2: Presentation of monohybrid crosses (one gene)

1. 1 Draw a 2x2 square grid. The simplest Pennett grating is very easy to do. Draw a large enough square and divide it into four equal squares. Thus, you get a table with two rows and two columns.
2. 2 In each row and column, label the parent alleles with letters. In a Punnett lattice, columns are for maternal alleles and rows for paternal alleles, or vice versa. In each row and column, write down the letters that represent the alleles of the mother and father. When doing this, use uppercase letters for dominant alleles and lowercase letters for recessive ones.
   • This is easy to understand from the example. Suppose you want to determine the likelihood that a given couple will have a baby who can roll their tongue into a tube. You can designate this property in Latin letters R and r - an uppercase letter corresponds to a dominant allele, and a lowercase letter to a recessive allele. If both parents are heterozygous (have one copy of each allele), then you should write one "R" and one "r" above the hash and one "R" and one "r" to the left of the grill.
3. 3 Write the appropriate letters in each cell. You can easily fill in the Punnett grid after you understand which alleles come in from each parent. Write in each cell a combination of two-letter genes that represent the alleles from the mother and father. In other words, take the letters in the corresponding row and column and write them in this cell.
   • In our example, the cells should be filled as follows:
   • Top left cell: RR
   • Top right cell: Rr
   • Bottom left cell: Rr
   • Bottom right cell: rr
   • Note that the dominant alleles (capital letters) should be written in front.
4. 4 Determine the possible genotypes of the offspring. Each cell of the filled Punnett lattice contains a set of genes that are possible in a child of these parents. Each cell (that is, each set of alleles) has the same probability - in other words, in a 2x2 grid, each of the four possible choices has a probability of 1/4. The various combinations of alleles presented in the Punnett lattice are called genotypes... Although genotypes represent genetic differences, this does not necessarily mean that each variant will produce different offspring (see below).
   • In our example of a Punnett lattice, a given pair of parents may have the following genotypes:
   • Two dominant alleles (cell with two Rs)
   • One dominant and one recessive allele (cell with one letter R and one r)
   • One dominant and one recessive allele (cell with R and r) - note that this genotype is represented by two cells
   • Two recessive alleles (cell with two letters r)
5. 5 Determine the possible phenotypes of the offspring.Phenotype an organism represents actual physical traits that are based on its genotype. Examples of phenotypes include eye color, hair color, sickle cell disease, and so on - although all of these physical traits are determined genes, none of them is given by its own special combination of genes. The possible phenotype of the offspring is determined by the characteristics of the genes. Different genes manifest themselves differently in the phenotype.
   • Suppose in our example that the gene responsible for the ability to fold the tongue is dominant. This means that even those descendants whose genotype includes only one dominant allele will be able to roll the tongue into a tube. In this case, the following possible phenotypes are obtained:
   • Top left cell: can fold tongue (two Rs)
   • Top right cell: can fold tongue (one R)
   • Bottom left cell: can fold tongue (one R)
   • Bottom right cell: cannot collapse language (no capital R)
6. 6 Determine the likelihood of different phenotypes by the number of cells. One of the most common uses of the Punnett grid is to find the likelihood of a phenotype occurring in offspring. Since each cell corresponds to a certain genotype and the probability of occurrence of each genotype is the same, to find the probability of a phenotype, it is enough divide the number of cells with a given phenotype by the total number of cells.
   • In our example, the Punnett lattice tells us that for given parents there are four possible combinations of genes. Three of them correspond to a descendant that is capable of folding the tongue, and one corresponds to the absence of such an ability. Thus, the probabilities of two possible phenotypes are:
   • The descendant can collapse the language: 3/4 = 0,75 = 75%
   • The descendant cannot fold the tongue: 1/4 = 0,25 = 25%

Method 2 of 2: Introducing a dihybrid cross (two genes)

1. 1 Divide each cell of the 2x2 grid into four more squares. Not all gene combinations are as simple as the monohybrid (monogenic) crossing described above. Some phenotypes are defined by more than one gene. In such cases, all possible combinations should be taken into account, which will require bOLarger table.
   • The basic rule of thumb for applying the Punnett Lattice when there are more than one genes is as follows: for each additional gene, double the number of cells... In other words, for one gene, a 2x2 grid is used, for two genes, a 4x4 grid is used, for three genes, an 8x8 grid is used, and so on.
   • To make it easier to understand this principle, consider an example for two genes. To do this, we will have to draw a lattice 4x4... The method outlined in this section is also suitable for three or more genes - you just need bOBigger grill and more work.
2. 2 Identify the genes from the parents. The next step is to find the parental genes that are responsible for the trait you are interested in.Since you are dealing with multiple genes, you need to add another letter to the genotype of each parent — in other words, you need to use four letters for two genes, six letters for three genes, and so on. As a reminder, it is helpful to write the mother's genotype above the grid and the father's genotype to the left of it (or vice versa).
   • For illustration, consider a classic example. The pea plant can have smooth or wrinkled grains and the grains can be yellow or green in color. The yellow color and smoothness of the peas are the dominant features. In this case, the smoothness of the peas will be denoted by the letters S and s for the dominant and recessive gene, respectively, and for their yellowness we will use the letters Y and y. Suppose a female plant has the genotype SsYy, and the male is characterized by the genotype SsYY.
3. 3 Write down the various combinations of genes along the top and left edges of the grid. Now we can write above the grid and to the left of it the various alleles that can be passed on to descendants from each parent. As with a single gene, each allele can be transmitted with the same probability. However, since we are looking at multiple genes, each row or column will have multiple letters: two letters for two genes, three letters for three genes, and so on.
   • In our case, it is necessary to write out various combinations of genes that each parent is able to transfer from his genotype. If the mother's genotype SsYy is on the top, and the father's genotype SsYY is on the left, then for each gene we get the following alleles:
   • Along the top edge: SY, Sy, sY, sy
   • Along the left edge: SY, SY, sY, sY
4. 4 Fill in the cells with the appropriate allele combinations. Write letters in each cell of the lattice in the same way as you did for one gene. However, in this case, for each additional gene, two additional letters will appear in the cells: in total, in each cell there will be four letters for two genes, six letters for four genes, and so on. As a general rule, the number of letters in each cell corresponds to the number of letters in the genotype of one of the parents.
   • In our example, the cells will be filled as follows:
   • Top row: SSYY, SSYy, SsYY, SsYy
   • Second row: SSYY, SSYy, SsYY, SsYy
   • Third row: SsYY, SsYy, ssYY, ssYy
   • Bottom row: SsYY, SsYy, ssYY, ssYy
5. 5 Find phenotypes for each possible offspring. In the case of several genes, each cell in the Punnett lattice also corresponds to a separate genotype of possible offspring, it is just that there are more genotypes than with one gene. And in this case, the phenotypes for a particular cell are determined by which genes we are considering. There is a general rule according to which for the manifestation of dominant traits it is sufficient to have at least one dominant allele, while for recessive traits it is necessary that all the corresponding alleles were recessive.
   • Since smoothness and yellowness of the grains are dominant for peas, in our example, any cell with at least one capital letter S corresponds to a plant with smooth peas, and any cell with at least one capital letter Y corresponds to a plant with a yellow grain phenotype. Plants with wrinkled peas will be represented by cells with two lowercase s alleles, and in order for the seeds to be green, only lowercase y are needed. Thus, we get the possible options for the shape and color of the peas:
   • Top row: smooth / yellow, smooth / yellow, smooth / yellow, smooth / yellow
   • Second row: smooth / yellow, smooth / yellow, smooth / yellow, smooth / yellow
   • Third row: smooth / yellow, smooth / yellow, wrinkled / yellow, wrinkled / yellow
   • Bottom row: smooth / yellow, smooth / yellow, wrinkled / yellow, wrinkled / yellow
6. 6 Determine the probability of each phenotype in the cells. To find the likelihood of different phenotypes in the offspring of a given parent, use the same method as for a single gene.In other words, the probability of a particular phenotype is equal to the number of cells corresponding to it divided by the total number of cells.
   • In our example, the probability of each phenotype is:
   • Offspring with smooth and yellow peas: 12/16 = 3/4 = 0,75 = 75%
   • Descendant with wrinkled and yellow peas: 4/16 = 1/4 = 0,25 = 25%
   • Offspring with smooth and green peas: 0/16 = 0%
   • Descendant with wrinkled and green peas: 0/16 = 0%
   • Note that the inability to inherit the two recessive alleles y has resulted in no possible offspring with green seed plants.

Tips

• Are you in a hurry? Try using an online Punnett Lattice Calculator (like this one), which fills in the Lattice cells for your given parental genes.
• As a rule, recessive signs are less common than dominant ones. However, there are situations in which recessive traits can increase the adaptability of the organism, and such individuals become more common as a result of natural selection. For example, a recessive trait that causes a blood disorder such as sickle cell anemia also increases resistance to malaria, which is beneficial in tropical climates.
• Not all genes are characterized by only two phenotypes. For example, some genes have a separate phenotype for a heterozygous (one dominant and one recessive allele) combination.

Warnings

• Remember that each new parental gene doubles the number of cells in the Punnett lattice. For example, with one gene from each parent, you get a 2x2 grid, for two genes, a 4x4 grid, and so on. In the case of five genes, the size of the table would be 32x32!